QD method combines best of optical, electron microscopy

A fast, versatile and high-resolution technique that combines the best of optical and scanning electron microscopy could provide surface and subsurface viewing of features as small as 10 nm in size. This will be useful for a wide range of applications including materials characterization and the life sciences, its creators say.

Researchers at NIST have developed the microscopy method using cathodoluminescence to image nanoscale features. In an old tube television, a beam of electrons moves over a phosphor screen to create images; the new technique works in much the same way by scanning a beam of electrons over a sample that has been coated with specially engineered quantum dots (QDs).

A new microscopy technique developed at NIST works by scanning a beam of electrons over a sample that has been coated with specially engineered quantum dots. The dots absorb the energy and emit it as visible light that interacts with the sample at close range; the scattered photons are collected using a similarly closely placed photodetector (not depicted). Courtesy of Dill/NIST.
The QDs emit low-energy visible light very close to the surface of the sample, exploiting near-field effects of light. After interacting with the sample, the scattered photons are collected using a closely placed photodetector, allowing an image to be constructed.

The first demonstration of the technique was used to image the natural nanostructure of the photodetector itself. Because both the light source and detector are so close to the sample, the diffraction limit doesn’t apply, and much smaller objects can be imaged.

“Initially, our research was driven by our desire to study how inhomogeneities in the structure of polycrystalline photovoltaics could affect the conversion of sunlight to electricity and how these devices can be improved,” said Heayoung Yoon of the Energy Research Group at NIST. “But we quickly realized that this technique could also be adapted to other research regimes, most notably imaging for biological and cellular samples, wet samples, samples with rough surfaces, as well as organic photovoltaics.”

The technique tackles two problems in nanoscale microscopy: the diffraction limit, which restricts conventional optical microscopes to resolutions no better than about half the wavelength of the light (about 250 nm for green light), and the relatively high energies and sample preparation requirements of electron microscopy, which are destructive to fragile specimens like tissue.

NIST researcher Nikolai Zhitenev, a co-developer of the technique, had the idea a few years ago to use a phosphor coating to produce light for near-field optical imaging, but at the time, no phosphor available was thin enough. Thick phosphors cause the light to diverge, severely limiting the image resolution.

This changed when the NIST investigators teamed with scientists from a company that engineers QDs for lighting applications. The QDs potentially could do the same job as a phosphor and be applied in a coating both homogenous and thick enough to absorb the entire electron beam while also thin enough that the light produced does not have to travel far to the sample.

The collaborators discovered that the QDs’ unique core-shell design efficiently produced low-energy photons in the visible spectrum when energized with a beam of electrons. The group then developed a deposition process to bind them to specimens as a film with a controlled thickness of approximately 50 nm.

The investigators now would like to develop the method further, working with end users, Zhitenev said.

Worcester Polytechnic Institute, QD Vision, Sandia National Laboratories and the Maryland NanoCenter at the University of Maryland also contributed to the research, which appeared online in AIP Advances (doi: 10.1063/1.4811275).

Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.